Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.3.7.6
REX640
Technical Manual
GUID-8CB941C0-A958-46AF-A509-C10AEE534324 V1 EN
Figure 392: Operating characteristic for the stabilized stage of the generator
differential protection function

Application

The differential protection works on the principle of calculating the differential
current at the two ends of the winding, that is, the current entering the winding is
compared to the current exiting the winding. In case of any internal fault, the currents
entering and exiting the winding are different, which results in a differential current,
which is then used as a base for generating the operating signal. Due to this principle,
the differential protection does not trip during external faults. However, it should be
noted that interturn faults in the same phase are usually not detected unless they
developed into some other kind of fault.
The short circuit between the phases of the stator windings normally causes large fault
currents. The short circuit creates a risk of damages to the insulation, windings and
stator core. The large short circuit currents cause large current forces which can
damage other components in the machine. The short circuit can also initiate explosion
and fire. When a short circuit occurs in a machine, there is a damage that has to be
repaired. The severity and the repair time depend on the degree of damage, which is
highly dependent on the fault time. The fast fault clearance of this fault type is of
greatest importance to limit the damages and the economic loss.
To limit the damages in connection to the stator winding short circuits, the fault
clearance time must be as short as possible (instantaneous). The fault current
Section 4
Protection functions
709